1. Field of the Invention
The present invention relates to a compressor that is used for an exhaust gas turbocharger of an engine, the compressor including, but not limited to: a recirculation passage connecting an inlet slit that is opened toward the air passage on the casing of the impeller to an outlet slit that is opened toward the compressor inlet air passage, wherein a part of the air flow through the impeller is taken in the recirculation passage through the inlet slit whereas the air taken into the recirculation passage flows toward the compressor inlet air passage.
2. Background of the Invention
The compressor of the turbocharger used for a vehicle has a performance characteristic as shown in FIG. 12, in which the vertical axis and the lateral axis denote the pressure ratio of the compressor and the fluid flow rate; in FIG. 12, along a line of a constant rotation speed Ni (i=1, 2, . . . ), the smaller the flow rate, the higher the pressure ratio. Further, when the rotation speed Ni is increased (for Ni, i=1, 2, . . . ), the pressure ratio is apt to increase.
Further, when the flow rate is increased, there appears a choking phenomenon where the flow rate can be no longer increased; on the other hand, when the flow rate is decreased, there appears a surging phenomenon where an irregular flow such as a backward flow of the working air occurs and the operation of the compressor becomes impossible. Thus, there is an operation zone in which the compressor can be operated between the surging limit on the smaller flow rate side and the choking limit on the greater flow rate side.
Further, the compressor of the turbocharger used for a vehicle has to be operated in a wide range regarding the flow rate; thus, it is required to enhance the operation zone of the compressor as wide as possible. Hence, it becomes necessary that the surging limit line L1 (cf. FIG. 12) on the smaller flow rate side be shifted toward the left side as far as possible, in order that the operation zone is enhanced, the shifted surging limit line being depicted as the line L2 (in FIG. 12).
An approach called a casing treatment is known as one of the manners by which the operation zone of the compressor is enhanced. According to the casing treatment, the casing of the compressor is provided with a groove or a circulation flow passage so that the air flow in the compressor is controlled. According to an example of the casing treatment approaches, in a case of the smaller air flow rate, a part of the air entering the inside of the compressor is recirculated so that the apparent flow rate is increased; thus, the surging can be hard to happen and the operation zone can be enhanced.
However, according to the approach as described above, it is necessary to machine the inner surface of the casing so as to form the recirculation passage for the return of the flow; thus, the production cost increase is incurred.
For instance, as shown in FIG. 13, an impeller 03 is fitted to the outer periphery of an end side of the rotor hub 01 whereas a turbine (not shown) is provided on another end side of the rotor hub; the rotor hub 01 and the impeller 03 are rotated around a rotation axis 05 by the turbine. Further, the impeller 03 is housed in a compressor housing 07; an air inlet passage 09 is formed on the air inlet side of the impeller 03; a diffuser 11 is provided on the air outlet side of the impeller 03; a scroll (a compressor outlet part) 13 is provided on the downstream side of the diffuser 11.
A recirculation passage 015 of an annular geometry is formed at the casing of the impeller 03 in the compressor housing 07; an inlet slit 017 is formed so that the inlet side of the recirculation passage 015 communicates with the air passage on the casing of the impeller; the outlet side of the recirculation passage 015 is opened toward the air inlet passage 09 so that the air entering the impeller is circulated to the air inlet side of the impeller 03.
Further, as shown in FIG. 13, in the configuration in which the outlet side of the recirculation passage 015 is opened toward the air inlet side of the impeller 03, the sound generated in the impeller is easily transmitted toward the upstream side of the impeller; thus, there may be a problem that the noise level becomes higher.
In order to control the noise, a noise insulation cover can be provided; however, providing a noise insulation cover brings another problem that the production cost is increased.
On the other hand, Patent Reference 1 (JP2007-127108) as well as Patent Reference 2 (JP2007-127109) discloses a technology in relation to the recirculation passage and the anti-noise measures.
In the disclosure of Patent Reference 1 as shown in FIG. 14, the compressor including a recirculation passage 028 connecting an inlet slit 021 that is opened toward the air passage on the casing of the impeller 020 to an outlet slit 026 that is opened toward an air inlet passage 024 of the compressor 022, wherein a part of the air flow through the impeller 020 is taken in the recirculation passage 028 through the inlet slit whereas the air taken into the recirculation passage streams toward the air inlet passage 024 through the outlet slit 026.
A recirculation passage forming member 032 is provided so as to form the recirculation passage 028 at the outer periphery of the air inlet passage 024 inside of the compressor housing 030; the recirculation passage forming member 032 is detachably attached to the outer periphery of the recirculation passage 028 of the air inlet passage 024 in the compressor housing 030; and, the recirculation passage 028 and the outlet slit 026 are formed by the inner recirculation-passage-side surface of the recirculation passage forming member 032 and the inner surface of the compressor housing 030.
Further, in the disclosure of Patent Reference 2 as shown in FIG. 15, the compressor including a recirculation passage 048 connecting an inlet slit 041 that is opened toward the air passage on the casing of the impeller 040 to an outlet slit 046 that is opened toward an air inlet passage 044 of the compressor 022, wherein a part of the air flow through the impeller 040 is taken in the recirculation passage 048 through the inlet slit whereas the air taken into the recirculation passage streams toward the air inlet passage 044 through the outlet slit 046.
Thereby, the outlet slit 046 is formed so that the direction of the outlet slit 046 and the radial direction of the impeller 040 form a predetermined acute angle α (FIG. 15); namely, the center line regarding the air flow out of the outlet slit 046 is directed toward the impeller 040. In addition, the passage area of the outlet slit 046 is made greater than the passage area of the inlet slit 041. Further, the recirculation passage forming member 050 is provided so as to form the recirculation passage 048 at the outer periphery of the air inlet passage 044 inside of the compressor housing 052; the outer periphery surface of the recirculation passage forming member 050 and the inner surface of the compressor housing 052 form the recirculation passage 048 as well as the inlet slit 041.